1. Field Of The Invention
The field of this invention is plasma assisted chemical vapor deposition of diamond-like-carbon (DLC) coatings applied to individual substrates, such as integrated chips, or to linear substrates such as fibers, wires, tubes, and sheets. In particular, methods are given which produce DLC coatings that can protect substrates, can function as hermetic coatings, and can be applied to individual and linear substrates.
2. Information Disclosure Statement
Due to their hardness and wear resistance DLC films [A Grill, "Review of the tribology of diamond-like carbon,"Wear, 168 (1993) 143-153] are used as protective coatings with applications ranging from IR optics to cutting tools. In addition, because of their amorphous structure and lack of grain boundaries they can also act as hermetic coatings with applications ranging from vapor barriers coatings on plastics to protective coatings preventing the diffusion of small gas molecules into optical fibers. These advantages are particularly important when fibers are imbedded in an application without plastic jackets. An example would be optical fibers used in optical gyroscopes; as temperatures change DLC coatings keep the fiber from being damaged. Another usage is where tight windings for small, compact packaging is required, as in guided missile payout tethers.
A range of techniques have been developed to deposit these coatings, including magnetron sputtering, neutral atom beam, dc discharges, and radio frequency (rf) discharges. A. C. Evans, J. Franks and P. J. Revell, "Diamond-like carbon applied to bioengineering materials," Surface and Coating Technology, 47 (1991) 662-667, A Grill, V. Patel and B. Meyerson, "Tribological behavior of diamond-like carbon: effects of preparation conditions and annealing," Surface and Coatings Technology, 49 (1991) 530-536. Of these techniques, rf plasmas have been most widely used. The majority of work has been at 13.56 MHz, but work at lower frequencies, 2.3 to 3.75 MHG, has yielded similar results for given film compositions. Typical depositions employ hydrocarbon pressures ranging from 0.01 to 0.1 Torr and rf powers of about 1 W/cm.sup.2 over the cathode area. At higher pressures film quality deteriorates, i.e., hardness, wear resistance, and other useful properties are compromised.
Desirable film properties, such as hardness, scratch resistance and wear resistance, generally require that the film strongly adheres to the substrate. Plasma pretreatment has been reported in the literature as a technique to enhance film adhesion. In particular argon plasma etching has been used for substrate pretreatment before DLC film deposition. D. M. Grant, et. al., "Plasma assisted CVD for biomedical applications," Diamond and Related Materials, 1 (1992) 727-730. This pretreatment is limited, however, to pressures of less than 0.1 Torr. Experimental results at higher pressures, e.g., between 0.5 and 7.0 Torr, made in the present studies, demonstrated that argon pretreatment actually reduced film adhesion below that achieved using no argon pretreatment. Similar results were obtained by Bailey et al., U.S. Pat. No. 5,470,661, where they found that the use of helium instead of argon yielded more thermally stable DLC films. We attribute this to better adherence of the helium diluted films over the argon diluted ones.
The quality of DLC coatings can be assessed by measuring the hardness, scratch resistance and wear resistance of the film. It was discovered that these properties correlate with a film's refractive index, which can be obtained using ellipsometry. Low refractive index films, typically with n in the range 1.78 to 1.85 measured with a wavelength of 675 nm, are soft, have low wear resistance and are easily scratched. Higher index films, with n up to 2.45, are hard and wear resistant and are difficult to scratch.
Linear products such as optical fibers, capillary tubing, wires or sheets, present particular coating problems for the present state of the art. Coatings applied at or near atmospheric pressures are relatively soft. To produce DLC coatings with high hardness, low pressure must be used (high vacuum). This is emphasized in Bailey et al., for example, wherein all the improved depositions, even with helium as the diluting gas, were performed at 20 mTorr or 0.020 Torr. The description indicates the processes described in the patent could be up to about 1000 mTorr, i.e. about 1 Torr. These pressures are still low for the kind of throughputs necessary in most commercial applications.
Due to the difficult problem of obtaining relatively high vacuums in an in-line manufacturing system--e.g., when an optical fiber is drawn through a DLC deposition system--linear substrates must be coated at very slow deposition rates. The high vacuum requirement can cause the introduction of flaws, or the further opening of existing microscopic imperfections, on the substrate's surface as the linear product is drawn through a tight-fitting entry gate into the low-pressure deposition areas, or through multiple gates into increasingly lower pressure areas until the deposition area is reached. Thus the prior art even in Bailey et al. are operating at pressures below what would be good for commercial applications, particularly for linear products such as optical fibers.
Another feature common to current prior art is the use of high frequency sources for the rf fields assisting the plasma formation. In Ovshinsky et at, U.S. Pat. No. 4,663,183, the operating radio frequency is typically 13.56 MHz and is recommended as being from above 0.1 MHz, i.e. above 100 kHz. It has been found that frequencies well below the prior art minimum provide particular benefits in conjunction with the higher pressures described below leading to higher deposition rates than expected.